If it is possible to produce hematopoietic stem cells from pluripotent stem cells in vitro, or remove hematopoietic stem cells from the living body, and culture and amplify only hematopoietic stem cells while preventing neoplastic transformation, it will solve many transplant-related problems and expand applications of hematopoietic stem cell transplantation therapy, as well as enabling exploration of new tissue engineering, such as development of other organ-specific stem cell transplantation therapy. The following problems have been pointed out regarding hematopoietic stem cell production and amplification methods developed up to now:    1. Introduction of HoxB4 gene into pluripotent stem cells can produce hematopoietic stem cells, but also induces acute leukemia.    2. Introduction of HoxB4 gene into hematopoietic stem cells amplifies hematopoietic stem cells, but also induces acute leukemia.    3. Addition of cytokine induces differentiation of mature blood cells. As a result, it becomes difficult to maintain the undifferentiated state of hematopoietic stem cells.    4. Addition of a demethylating agent increases amplification efficiency of hematopoietic stem cells, but also affects epigenetics. Therefore, safety must be checked.
Theoretically, pluripotent stem cells can be differentiated into any cell lineage. However, no technique of determining the direction of cell differentiation in vitro has been established yet. Furthermore, no technique of preparing clinically applicable cells from hematopoietic stem cells has been established. Pluripotent stem cells have a property that once the cells differentiate into a lineage, the cells lose the ability to differentiate into other lineages. If it is possible to inhibit pluripotent stem cells from differentiating into cells other than hematopoietic stem cells, production of hematopoietic stem cells from pluripotent stem cells can be expected. Hematopoietic stem cells have apparently incompatible abilities, i.e., self-renewal ability and pluripotency. Differentiation of hematopoietic stem cells subjected to stimulation such as a cytokine produces mature blood cells. More specifically, to amplify hematopoietic stem cells, accelerating self-renewal while inhibiting differentiation into various cell lineages is necessary. To develop new hematopoietic stem cell production and amplification methods while overcoming the aforementioned problems, search for and development of the following factors are important: a factor that inhibits differentiation of cells other than hematopoietic stem cells, a factor that promotes only self-renewal of hematopoietic stem cells, and a factor that inhibits differentiation of hematopoietic stem cells. A fetal liver is a principal site for hematopoietic stem cell amplification in vivo. Accordingly, to search for and develop new factors, elucidation of the amplification mechanism of hematopoietic stem cells in the liver and reproduction of the results in a test tube, i.e., taking a orthodox approach, will be a quick way.
Patent Literature (PTL) 1 discloses a hematopoietic stem cell proliferator comprising a placenta-constituting cellular debris as an active ingredient. However, such a biologically derived material has problems, such as infection and difficulty in preparing the material.
Patent Literature (PTL) 2 discloses a factor for promoting the maintenance and amplification of hematopoietic stem cells, comprising a membrane protein Thsd1/Tmtsp. However, preparing such a membrane protein in an active form is difficult.